Biofuels production for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels is increasing worldwide. Such biofuels include, for example, ethanol, propanol, i-butanol and n-butanol. One of the major drivers for biofuels is their derivation from renewable resources by fermentation and bioprocess technology. Conventionally, biofuels are made from readily fermentable carbohydrates such as sugars and starches. For example, the two primary agricultural crops that are used for conventional bioethanol production are sugarcane (Brazil and other tropical countries) and corn or maize (U.S. and other temperate countries). The availability of agricultural feedstocks that provide readily fermentable carbohydrates is limited because of competition with food and feed production, arable land usage, water availability, and other factors. Consequently, lignocellulosic feedstocks such as forest residues, trees from plantations, straws, grasses and other agricultural residues may become viable feedstocks for biofuel production. However, the very heterogeneous nature of lignocellulosic materials that enables them to provide the mechanical support structure of the plants and trees makes them inherently recalcitrant to bioconversion.
One available technology path to convert lignocellulosic biomass to ethanol is to convert lignocellulosic biomass to syngas (also known as synthesis gas) in a gasifier and then ferment this gas with anaerobic microorganisms to produce biofuels such as ethanol, propanol, i-butanol and n-butanol or chemicals such as acetic acid, butyric acid and the like. This technology path can convert all of the components to syngas with good efficiency (e.g., greater than 75% of the energy content is recovered as carbon monoxide and hydrogen), and some strains of anaerobic microorganisms can convert syngas to ethanol, propanol, n-butanol or other chemicals with high (e.g., greater than 90% of theoretical) efficiency. Moreover, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, peat, petroleum coke, coal, solid waste, land fill gas and biogas, and be obtained as an off gas from other industrial processes, making this a more universal technology path.
However, production of syngas from carbonaceous feedstocks can result in the generation of hydrogen cyanide as a contaminant that is detrimental to the biological conversion of the syngas to oxygenated organic compound. Hydrogen cyanide must be removed from syngas and then managed or destroyed in an environmentally acceptable manner, generally at significant expense.
Since the syngas may contain undesired contaminants in addition to hydrogen cyanide, methods for treating the syngas to remove hydrogen cyanide must also take into account other components of the syngas, some of which may be beneficial or substantially inert in the fermentation process and some of which may be detrimental to the microorganisms.
Numerous processes are known for removing hydrogen cyanide from gases. Conventional methods for removal of hydrogen cyanide from syngas prior to its use generally involves scrubbing with aqueous solutions to remove these compounds from the syngas with subsequent discharge of the scrubbing solutions to wastewater treatment or via alternate disposal methods. Other methods include catalytic sorption, adsorption, condensation with aldehydes, and reaction with metal cations such as iron, cobalt, nickel, copper and the like. See, for instance, United States Published Patent Application No. 20110097701 A1, and U.S. patent application Ser. No. 13/304,902, filed on Nov. 28, 2011, disclosing the use of peroxygenated reactants, both hereby incorporated by reference in their entirety. Van Dyk, et al., in U.S. Pat. No. 8,128,898 B2, disclose processes for the removal of hydrogen cyanide from synthesis gas using a scrubbing solution containing at least one dissolved metal salt. The metal cations are capable of forming metal cyanide complexes or precipitates and include one or more of iron, copper, cobalt, silver, gold or other transition metal cations. A buffer is used to maintain the scrubbing solution in a pH range of between 6 and 10. The salts are preferably present in a concentration in the scrubbing solution at about 1 to 20 mass percent. Haese in U.S. Pat. No. 3,950,492, discloses processes for removing hydrogen sulfide and hydrogen cyanide from gases using an aqueous iron salt washing solution formed from at least one acid selected from the group consisting of sulfuric acid and sulfurous acid.
Banfalvi in “Removing cyanide from waterways”, Chemscripts, October 2000, Vol. 30, No 10, 53-55, reviews several methods for removal of cyanide, e.g., from mining extraction operations, from rivers. Conversion of cyanide to thiocyanide was dismissed by the author since the conversion requires energy. The use of iron salts was found to be problematic as ferricyanide complexes can be toxic to some aquatic life, and the use of chlorine to generate oxygen for converting hydrogen cyanide to cyanic acid is unacceptable due to the toxicity of chlorine. The author proposes the use of carbon dioxide to generate carbonic acid and release hydrogen cyanide from cyanide salts which can evaporate or be oxidized to cyanic acid.
For an oxygenated organic compound fermentation process to be commercially viable, capital and operating costs must be sufficiently low that it is at least competitive with alternative biomass to oxygenated organic compound processes. For instance, ethanol is currently commercially produced from corn and cane sugar in facilities having name plate capacities of over 100 million gallons per year at sufficiently low costs to be competitive with fossil fuels. Commercial-scale bioreactors thus often have capacities of greater than 1 million, and more frequently greater than about 5 or 10 million, liters in order to capture economies of scale.
Although methods are well known for removing hydrogen cyanide from gases, the operation of commercial-scale fermentation facilities to convert the syngas to an oxygenated organic compound cannot tolerate a failure of the hydrogen cyanide removal unit operation due to the adverse impact of the failure on the population of microorganisms in the fermentation broth. In general, microorganisms used for the anaerobic bioconversion of syngas to oxygenated organic compound are sensitive to the presence of hydrogen cyanide and often hydrogen cyanide concentrations in the fermentation broth must be maintained below about 100, and sometimes less than 10, parts per billion mass per volume. The economic cost of a failure of the hydrogen cyanide removal unit operations on such a commercial process can readily be appreciated when it is realized that repopulating a bioreactor with microorganisms to achieve steady-state operation may take 5 to 10 days. Moreover, if the fermentation broth has to be discarded due to the presence of hydrogen cyanide, losses of nutrients and other adjuvants would be incurred.
The use of redundant hydrogen cyanide removal unit operations to avoid a break through or address a failure of the hydrogen cyanide removal unit operation adds to the capital and operating expense of facility. Accordingly, processes are sought to protect the population of microorganisms in an anaerobic bioreactor used to convert syngas to oxygenated organic compound in the event of a hydrogen cyanide break through or failure of the hydrogen cyanide removal unit operation, which processes are not only effective but also do not require undue capital and operating expense to implement. Most desirably such processes would be in situ processes in the fermentation broth to avoid an additional unit operation dedicated solely to hydrogen cyanide removal. However, such processes must be effective without adversely affecting the microorganisms or the bioconversion of syngas to oxygenated organic product.